A modern petroleum refinery is designed to maximize the production of select liquid products from crude oil. In addition to the well known atmospheric and vacuum distillation processes used to provide refined products, many refineries utilize petroleum cokers to further process the residual materials remaining after distillation. The three common coking processes, fluid, flexi and delayed coking, have been used for decades. As such the common operating conditions for petroleum coking are well known throughout the industry.
During the fill cycle of the coking process, a foam layer forms on the surface of the feedstock as it fills the coke drum. Operators must control foaming within the coke drums otherwise the foam will enter the overhead vapor line resulting in a blockage.
Management of the silicone anti-foam (AF) agent injection is critical as any carry-over of the silicone material through the overhead vapor line will poison the catalyst found in downstream operating units such as the hydrotreating unit. Thus, operations which use too little silicone based AF agent may foam-over and carry the silicone downstream. However, excessive usage of silicone based AF agent, due to continuous injection, increases costs, may reduce the production of valuable liquids and may lead to an undesirable coke material.
Delayed coking reactions cause foaming in the coke drums which if uncontrolled can carry heavy tars and coke beyond the coke drum into piping and the distillation system. An uncontrolled foam-over will render the piping and fractionator in the coker inoperable and require a shutdown of the unit for cleaning and repair of any damaged mechanical elements. This is very costly and operators of delayed cokers avoid it by suppressing the foam front in the coke drum that forms during the thermal conversion of coker feedstock to coke and a range of vapors.
Foam suppression is typically accomplished by injection of high molecular weight silicone material in the form of polydimethylsiloxanes (PDMS) into the coke drum. The PDMS breaks down due to the high temperature in the coke drum and most of the cracked PDMS products vaporize and carry over and contaminate the hydrocarbon liquids recovered in the downstream fractionator. The contamination causes catalyst poisoning in refining units used to further process the coker liquids to finished products.
Coke drums are used to provide the residence time required for completion of the thermal reactions in a batchwise mode with a continous feed of hot feedstock. When the coke drum is filled, the hot feedstock is switched to another coke drum that has been prepared to receive it. To minimize the amount of PDMS used, it is typically injected only in the latter part of the coke drum fill cycle and during a few subsequent operations when foaming and reactant liquids are closest to the coke drum outlet. During these latter stages of the drum cycle, the drum may experience pressure surges. A sharp, small reduction in pressure can result in a significant increase in foam height risking a foamover. This is particularly true when there has been even a small reduction in the internal temperature of the coke drum.
Foaming is caused by higher surface tension and viscosity of the partially converted liquids in the coke drum and drum vapors blowing through the liquid. Common ways to reduce the risk of foam-overs and the usage of PDMS are:                1. Providing a higher vapor space in the coke drum when the coke drum fill cycle is complete. This can have negative operating cost implications or require unit modifications.        2. Increasing the temperature of the feed to the coke drum to reduce the surface tension and viscosity of the partially converted liquid reactant mass. The reaction heat is supplied by an upstream fired heater that may be limited in capability to operate at higher temperatures effectively and economically. In some cases there are undesirable process consequences to raising the coking temperature which can affect coke product properties.        3. Adding more aromatic oils to the feedstock. This requires that the added oil, commonly clarified slurry oil (CSO) from a fluid catalytic cracking unit, be provided in enough volume to beneficially affect the properties of the drum liquid. A commonly used material is called decant oil or clarified slurry oil (CSO). If too much is required an undesirable recycle of unconverted CSO can be formed between the delayed coker and the Fluid Catalytic Cracking Unit (FCCU).        
PDMS is delivered to the refiner as a liquid diluted with hydrocarbon liquid usually with kerosene properties. Distribution of this antifoam liquid into the coke drum is typically accomplished by further dilution of the antifoam in a carrier oil, commonly a light and/or heavy gas oil fraction produced by the delayed coker.
Foam is produced by the actions described above and since the feed and vaporization occurs continuously through the coking cycle, the foam is continuously replenished as the foam bubbles drain. PDMS changes the liquid properties in the foam causing it to drain faster resulting in a reduced height of the froth.
Further improvements are desired in the current coking process. In particular, improved processes which control foaming in order to enhance liquid yield while improving the coke material are desired. Further, a coking foam control method which reduces the amount of silicone based AF agent used will particularly enhance the coking process.